most likely cause for hotmail sending out spam to all my contacts?

Ive noticed that once a month, feb, april, and may, my hotmail email has been sending out spam to all my contacts. It even appears in the hotmail sent folder. I rarely use my hotmail email address, and have never had a similar issue before with my other email addresses (that I know of-eg Ive never sent spam to my contacts which has also appeared in the 'sent' folder on yahoo etc). I have changed my hotmail password as a result, and scanned this machine fully/kept it updated with adaware, avira, malwarebytes, spywareblaster. Is it likely to be an infection on the pc, or the result of some automated system working out my password and using my hotmail on its own to send out its own spam? Im not 100% on how it works exactly

They seem to just be sending out advertising to my contacts, surely for these crackers obtaining emails with saved passwords (eg bank details) would be the main target.. but then perhaps that happens too. Thats not to say that I actually save these details, but many I know do! I think Ill just have to stop using hotmail altogether if its the main target. Ill have to think up far better passwords for my other non hotmail accounts, but then symbols are not always accepted, which can make it awkward to create a difficult password to crack, but then it would also make it difficult for many to remember ;( my memory is not the best! :) thanks

I think it was woodchip who suggested a workaround for this sort of problem, just add a ! or similar to the beginning of each of your contacts email addresses and then when you want to send mail to them edit the address to remove the !

Then your contacts won't get any spam supposedly from you. You will know if your account has been compromised as all the emails will be 'undeliverable' and sent back to you as such.